I have a brass 23x152B fired Case headstamped “606” at 12 O’Clock, and date “BC-77” at 6 O’Clock. There is also a strange marking at 3 O’Clock resembling a vertical dash between the 2 dots of a colon. This headstamp is raised within the central indented circle as with 14.5x114mm headstamps. I am told brass 23x153B Cases with post WW2 dates were used in a subcalibre training device, which gun was it used with? What country and arsenal does this case originate from? Unfortunately some idiot took a hand drill to the primer. Thanks in advance for your help.
EDIT: Took headstamp photo earlier, thought I may as well post it for future reference. I had to leave it quite big so it can be clearly seen.